A nutcase? (7)
I believe the answer is:
cranium
'a nutcase?' is the definition.
(nut can mean one's head)
'a nutcase?' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for cranium that I've seen before include "hard case" , "Brain container" , "Part of the skull" , "Brainbox" , "Skull of a vertebrate" .)